#include<iostream>

using namespace std;

class Sum{
  public:
     Sum()
     {
        _ret+=_i;
        _i++;
     }
     int static GetRet()
     {
        return _ret;
     }
  private:
      static int _i;
      static int _ret;
};

int Sum::_i = 1;
int Sum::_ret = 0;

int main()
{
  int n = 0;
  cin>>n;
  Sum a[n];
  cout<<Sum::GetRet() <<endl;
  return 0;
}


